An atmospheric pressure sensor includes an air-tightly closed casing for measurement of an absolute pressure. A sensor unit is located in the casing. The sensor unit holds data of a reference pressure inside the casing. The volume of the casing changes in response to a variation in the atmospheric pressure. The sensor unit is designed to calculate the pressure inside the casing based on the change in the volume. The sensor unit utilizes a difference between the reference pressure and the detected pressure to measure the atmospheric pressure.
The pressure inside the air-tightly closed casing of the atmospheric pressure sensor is maintained at a constant level so as to ensure the accuracy of the measurement. A high pressure always acts on the sensor unit. The sensor unit sometimes suffers from a so-called creep. The creep is supposed to result in a deteriorated accuracy of the measurement. Moreover, the casing has to be air-tightly closed without any gap so as to maintain the pressure inside the casing at a constant level. A high processing technique is required to produce such a casing.